1. FIELD OF THE INVENTION
The present invention relates to a degassing apparatus for a metal mold which degasses a cavity of the metal mold during injection molding by an injection molding apparatus, a die-cast machine or the like.
2. DESCRIPTION OF THE PRIOR ART
In a conventional injection molding apparatus, for example, a die-cast machine, when molten metal is charged in the mold cavity at high speed and high pressure, gas in the mold cavity cannot often be sufficiently removed and may become mixed with the molten metal used to form a product, thus forming a void in the molded product.
U.S. Pat. No. 4,431,047 ('047 patent) describes a degassing apparatus for a metal mold which can remove a large volume of gas within a short period of time. The degassing apparatus comprises a vent groove formed on the dividing or mating surfaces of the metal mold. The vent groove communicates with the mold cavity.
The apparatus described in the '047 patent also includes a valve having a reciprocatingly movable valve body, and a bypass conduit which provides a gas exhaust path from the mold cavity. The bypass conduit joins the vent groove midway along its length and connects with the valve. The valve body may be moved between an open and closed position. In the open position, the valve body allows gas from the bypass conduit to pass freely through the valve. In the closed position, the valve body blocks the bypass conduit and the vent groove so that no molten metal may pass into the valve. The valve body is positioned in line with the vent groove at an end of the groove that is distal from the mold cavity.
The degassing apparatus of the '047 patent allows gas in the mold cavity to escape during injection molding through the bypass conduit and the valve. When molten metal is charged in the cavity and reaches the end of the vent groove, the molten metal has sufficient mass to push the valve body from the open position to the closed position, whereupon the bypass conduit and vent groove are closed so that no molten metal can escape.
Although the degassing apparatus described in the '047 patent works satisfactorily in many applications, it has a number of inherent drawbacks.
One of the problems when injection molding with molten metal is that, like similar liquids, it travels in waves or splashes, so that its flow may be discontinuous. This is especially true when splashes or droplets of molten metal are entrained by the air escaping the mold cavity, which droplets comprise the leading wave of molten metal flow impinging the valve body.
The droplets of molten metal may not have sufficient mass to move the valve body completely from the open position to the closed position. In fact, a compression spring biasing the valve body towards the open position may reopen the valve after the first droplets of molten metal have impinged the valve body and upon the arrival of further gas escaping from the mold cavity through the bypass conduit.
When a subsequent, more continuous wave of molten metal reaches the valve body, the first leading droplets may have already begun to solidify within the vent groove and may form a constriction within the vent groove. Thus, the "after wave" of molten metal may not be capable of impinging the valve body with sufficient kinetic force to fully close the valve. As a result, molten metal may enter the valve through the bypass conduit.
U.S. Pat. No. 4,489,771 ('771 patent) discloses a degassing apparatus having a valve and a valve body, a tension spring for biasing the valve body in a closed position, and a leaf spring or a compression spring, a ball and a cooperating detent formed in the valve body for releasably locking the valve body in an open position. When molten metal impinges the valve body, it overcomes the pressure of the spring and ball, which is seated in the detent, and frees the valve body to move to the closed position, thus closing the valve. The valve can be quickly and reliably closed when the first wave of molten metal impinges the valve body.
An inherent disadvantage of the degassing apparatus of the '771 patent is that it relies on the mechanical forces between the biased ball and valve body detent to maintain the valve body in an open position. Such mechanical forces may vary as the leaf spring biasing the ball weakens, or due to undue wear on the ball or the detent in which the ball is seated. The valve may close prematurely as the locking mechanism weakens, which may prevent gas from the mold cavity from escaping.
Alternatively, if the leaf spring or other components of the locking mechanism become roughened or deformed, an unusually high kinetic force may be needed to dislodge the ball from the valve body detent. The molten metal impinging the valve body may not have sufficient kinetic force to close the valve, thus allowing the metal to flow into the valve.
Thus, the degassing apparatus of the '771 patent may not be able to withstand the strenuous and continual degassing operation that is required in mass production injection molding processes, and may have an unusually short operational life due to its mechanical wear.